1. Field of the Invention.
The present invention relates generally to auxiliary or attachment hydraulic systems for mobile equipment. In particular, the present invention is a pressure relief hydraulic system for couplers which interconnect the hydraulic motor of the attachment to the vehicle's auxiliary hydraulic system.
2. Description of the Prior Art.
Mobile, off-road vehicles such as skid steer loaders are often equipped with auxiliaries or attachments which include a hydraulic motor. These attachments are supplied with hydraulic fluid by the vehicles auxiliary hydraulic system. A known prior art auxiliary hydraulic system 10, which can be embodied as a portion of an overall skid steer loader hydraulic system, is illustrated generally in FIG. 1.
Hydraulic system 10 includes a hydraulic pump 12 which is driven by engine 14. Hydraulic fluid from reservoir 16 is pressurized by pump 12 and provided to electro-hydraulic spool valve 18 through load check valve 20. Check valve 20 can be integral with valve 18. Valve 18 is an electro-hydraulically actuated open center valve which includes solenoid valves 24 and 25 and a spool (not separately shown) biased to its neutral position by springs 22. Attachment 26, which can be an auger, trencher, angle boom or other hydraulically powered implement, is coupled to valve 18 of hydraulic system 10 by mated sets of quick couplers 28.
An electrical system associated with hydraulic system 10 includes battery 30, ignition switch 32, ignition system 34, and a starter relay 36. Ignition switch 32 has a rotating contact 38, run contact 40, and start contact 42. A negative (-) terminal of battery 30 is connected to ground 43, while a positive (+) terminal is connected to rotating contact 38. Run contact 40 is coupled to ignition system 34, to ground 43 through auxiliary forward switch 44 and solenoid valve 24, and to ground 43 through auxiliary reverse switch 46 and solenoid valve 25. Hydraulic fluid pilot pressure source 48 is also coupled to solenoid valves 24 and 25. Pilot pressure sources such as 48 are well known and can include a separate charge pump or a return fluid flow from valve 18. Pressure in conduit 50 is maintained by relief valve 49, which allows return flow from valve 18 to be used as a pilot pressure source.
When rotated to its START position, ignition switch contact 38 will be interconnected to contacts 40 and 42. Battery 30 is thereby connected to both ignition system 34 and starter relay 36. Once engine 14 is started, contact 38 is rotated to its RUN position to continue current flow to ignition system 34. With switch contact 38 in its RUN position, and engine 14 running, an operator can use valve 18 to control the flow of hydraulic fluid provided to attachment 26. When forward switch 44 is actuated and closed, solenoid valve 24 will be energized and drive the valve spool from its neutral position against the force of spring 22, as pilot pressure from source 48 is introduced to fully stroke the spool, causing hydraulic fluid from pump 12 to be supplied to attachment 26 in a first or forward direction. The spool will return to its neutral position, thereby discontinuing fluid flow to attachment 26, when switch 44 is opened. In a similar manner, hydraulic fluid is provided to attachment 26 in a second or reverse direction when reverse switch 46 is actuated. Check valve 20 is included to prevent any backwards fluid flow from attachment 26 which might otherwise be caused by the force of gravity acting on attachment 26 as the spool the stroked position. Valve 18 is schematically shown in FIG. 1 to include transitional spool timing which occurs as the spool is moved from the neutral position to the fully stroked position.
A common problem with auxiliary hydraulic systems such as 10 is that pressurized hydraulic fluid can be trapped in the conduits between couplers 28 and valve 18 after ignition switch 32 is switched to its OFF position. This trapped hydraulic fluid pressure can make it impossible to use couplers 28 to interconnect attachment 26 unless expensive connect-under-pressure type couplers are used. Since engine 14 has stopped, no pilot pressure is available to actuate valve 18 and relieve the trapped pressure. Although the inability to move the valve spool after engine 14 has stopped is not a problem with valve spools which are actuated manually or purely electrically, this deficiency is inherent in state-of-the-art electro-hydraulic spool valves.
It is evident that there is a continuing need for improved auxiliary hydraulic systems. In particular, an auxiliary hydraulic system which includes an electro-hydraulic spool valve and does not require the use of connect-under-pressure couplers is desired.